The Year Without Summer by William K. Klingaman, Nicholas P. Klingaman Read Free Book Online

effective barrier. Should the vortex weaken, the pressure rises near the poles
     and falls in the middle latitudes, leading to frequent outbreaks of polar air. In
     the Northern Hemisphere, scientists have defined the North Atlantic Oscillation index
     to describe this seesaw of pressures between the poles and the middle latitudes, with
     a high index associated with a strong vortex.
    Because the aerosol cloud from Tambora heated the stratosphere in the middle latitudes,
     but not in the Arctic, it enhanced the stratospheric westerly winds around the polar
     vortex. This effect soon made its way from the stratosphere to the troposphere, strengthening
     the barrier to Arctic air and leading to a stronger than normal high-pressure system
     in the Atlantic Ocean near the Azores Islands. The unusually warm winter throughout
     New England likely resulted from fewer incursions of polar air into the region. Data
     from tree rings and other proxies for temperature indicate that the average winter
     temperature in 1815–16 was as much as three degrees Fahrenheit warmer than normal
     in a band extending southwest from Alaska through central and southern Canada, across
     the Great Lakes, and into New England.
    By strengthening the polar low and the Atlantic high-pressure system, the aerosol
     cloud also accelerated the trans-Atlantic westerly jet stream that steers weather
     systems from North America towards Europe. The jet stream also shifted north, bringing
     more systems to central and northern Europe and fewer to the Mediterranean Sea and
     North Africa. The westerly inflow of air from the Atlantic provided a steady source
     of moisture for these systems, which released that moisture over Europe in a series
     of snow- and rainstorms. The aerosol cloud effectively increased the North Atlantic
     Oscillation index; as weather forecasters are well aware, high values of this index
     are often associated with stormy winters across northern and central Europe. Using
     climate models to simulate the effects of past volcanic eruptions, scientists have
     found a consistent link between large eruptions and increases in the index the following
     winter, with the models producing a nearly constant stream of storms across the Atlantic
     as a result. The unsettled conditions across Europe in the winter of 1815–16 were
     likely the result of the aerosol cloud’s effect on the North Atlantic Oscillation.
    Although the primary effect of the aerosol cloud was to cool global temperatures,
     its strengthening of the wintertime Arctic vortex delayed the appearance of severely
     cold temperatures in the United States. Once the long, polar winter night ended, however,
     the vortex weakened. Sunlight returned to the Arctic, and the aerosol cloud began
     to heat the stratosphere there as well as at lower latitudes. The westerly wind barrier
     around the vortex largely vanished, and cold air became free to move away from the
     pole—south, towards the United States and Europe. The cooling effects of the aerosol
     veil again became dominant, setting the stage for a chilling spring and a disastrous
     summer.
    Nevertheless, the short-term effect of the mild winter of 1815–16 in the United States
     was to fuel the ongoing debate over whether American winters were growing warmer.
     Renowned Puritan cleric and naturalist Cotton Mather had first advanced this hypothesis
     in the late seventeenth century, less than a hundred years after the first English
     settlers arrived in Massachusetts Bay. “Our own Winters are, observably as Comfortably
     Moderated since the Land has been Peopled, and Opened, of Late Years,” wrote Mather.
     “Our Snows are not so Deep, and Long … and our Winds blow not such Rasours, as in
     the Days of our Fathers when the Hands of the Good Men would Freeze unto the Bread
     upon their Tables.” (Occasionally Mather veered into flights of hyperbolic excess
     in describing the rigors of winters past; he once claimed that when his